The invention relates to a photoconductive switch for switching electrical signals.
Switches for switching high-frequency electrical signals, such as microwave signals, need to have a low insertion loss in their ON state, and need to provide a high isolation in their OFF state. A photoconductive switch is capable of providing such characteristics. The conduction state, i.e., ON or OFF, of a photoconductive switch is controlled by incident light. A photoconductive switch has less interaction between the control signal and the signal being switched than an electrically-controlled switch such as a transistor. Moreover, a photoconductive switch has good switching performance for high-frequency signals.
U.S. Pat. No. 3,917,943 of Auston discloses a photoconductive switch (PCS) controlled by an ultra-short optical pulse and fabricated on a semiconductor substrate. Two gold micro-strip transmission lines separated by a narrow gap are located on the surface of a light-absorbing, semi-insulating semiconductor substrate. A first optical pulse directed at the substrate through the gap turns the PCS ON by generating copious electric charges near the substrate surface. A second optical pulse that begins during the first optical pulse and that is directed at the gap generates copious electric charges in the bulk of the substrate extending down to the ground plane. This shorts the micro-strip transmission lines to ground, and switches the PCS OFF.
The substrate is grown at a low temperature or is ion implanted to shorten the carrier lifetime to provide a fast switching response. However, these measures also reduce the carrier mobility, which causes the PCS to have a high insertion loss. In addition, the PCS requires a long time to reach its ultimate high resistance state after the second optical pulse has been asserted.
U.S. Pat. No. 4,755,663 to Derkits, Jr. indicates that a disadvantage of the Auston PCS is that the electrical impulse created by the optical pulse is dominated by carrier recombination, rather than carrier transport. Derkits discloses a PCS in which the portion of the substrate adjacent the gap includes a region composed of a graded-composition photosensitive semiconductor material having a grooved surface. Illuminating the substrate surface through the gap causes the PCS to conduct. The intensity of the light has to be sufficient to generate charge carriers at the surface of the photosensitive semiconductor material. Extinguishing the light turns the PCS OFF by generating a quasi-electric field that sweeps the charge carriers into a region of the substrate where a narrow band-gap energy material is predominant. In this region, the grooves separate the charge carriers and prevent further conduction between the electrodes.
However, the PCS disclosed by Derkits does not offer a sufficient performance improvement to enable it to meet present-day performance demands.
FIG. 1 shows a PCS 1 disclosed by two of the inventors of the present disclosure in U.S. patent application Ser. No. 09/337,045, assigned to the assignee of the present invention. This PCS has a low insertion loss in its ON state, a large isolation in its OFF state and is simple to fabricate. In the PCS 1, a thick, light-absorbing photoconductive layer of a semiconductor material having a narrow band-gap energy (NB material) is sandwiched between two thin confinement layers of a semiconductor material having a wide band-gap energy (WB material). The photoconductive switch 1 is composed of the substrate 13 on which are located the second confinement layer 12 of a WB material, the photoconductive layer 11 of an NB material and the first confinement layer 10 of a WB material. The electrodes 2 and 3 are located on the surface of the first confinement layer 10 and are separated from one another by the gap 4. The WB material of the first confinement layer 10 is doped n-type, and that of the photoconductive layer 11 is doped p-type.
Incident light 14 generated by a semiconductor light source (not shown), such as a laser, in response to an electrical control signal switches the PCS 1 ON. Switching OFF the incident light switches the PCS OFF.
The PCS 1 satisfies the basic performance requirements of having a low ON resistance and low OFF capacitance, but its dynamic switching response is not ideal. The dynamic switching response of the PCS 1, when tested using the test arrangement shown in FIG. 2A, exhibited the tailing characteristic illustrated in FIG. 2B.
In the test arrangement shown in FIG. 2A, the PCS 1 is connected in series between the DC power supply 21 and the load 22. The input of the oscilloscope 23 in connected in parallel with the load. The time base of oscilloscope is triggered by the electrical control signal (not shown) of the PCS. The dynamic switching response of the PCS 1 is observed using the oscilloscope.
The electrical control signal changing from its 0 state, in which the PCS 1 is OFF, to its 1 state, in which the PCS is ON, caused the PCS 1 to turn ON rapidly, as shown at 24 in FIG. 2B. The electrical control signal changing from its 1 state to its 0 state caused the PCS 1 to turn part-way OFF rapidly, as shown at 25. The PCS 1 then continued to turn OFF much more slowly than the initial rate 25, as shown at 26, and was still in a partly-ON state after as long as 100 xcexcs. The prolonged, gradual turn-off response shown in FIG. 2B will be called a tailing characteristic in this disclosure. A tailing characteristic is normally undesirable in the switching operation of a switch. Therefore, the need exists in the PCS just described to eliminate the tailing characteristic without degrading the low ON resistance, the low OFF capacitance and other desirable aspects of the performance of the PCS.
The invention provides a photoconductive switch that comprises a first confinement layer, a second confinement layer, a photoconductive layer that includes a doped sub-layer and an undoped sub-layer, a first electrode and a second electrode. The first confinement layer is a layer of a first semiconductor material having a first band-gap energy and a first conductivity type. The second confinement layer is a layer of a second semiconductor material having a second band-gap energy. The photoconductive layer is a layer of a third semiconductor material having a third band-gap energy and a second conductivity type, opposite to the first conductivity type. The photoconductive layer is sandwiched between the first confinement layer and the second confinement layer, and the third band-gap energy is less than the first and second band-gap energies. In the photoconductive layer, the doped sub-layer is in contact with the first confinement layer, and the undoped sub-layer is adjacent the second confinement layer. The first electrode and the second electrode are separated from each other by a gap, and are located on the surface of the first confinement layer remote from the photoconductive layer.
The doped sub-layer may be a first doped sub-layer, and the photoconductive layer may additionally include a second doped sub-layer in contact with the second confinement layer.
The photoconductive switch may additionally comprise a graded composition layer or a chirped super lattice multi-layered film sandwiched between the photoconductive layer and the first confinement layer.
The sub-layers of the photoconductive layer substantially eliminate the above-mentioned tailing characteristic from the dynamic switching response of the photoconductive switch without significantly increasing the capacitance and resistance of the photoconductive switch.